(1) Field of the Invention
The present invention relates to an EL element driving circuit and an EL element lighting device using the same, and, more specifically, relates to an EL element driving circuit in a lighting device using EL elements (EL element lighting device) in which the brightness of EL elements can be adjusted product by product. Thereby the variation in their illumination intensities of the products can be suppressed and the power consumption thereof can be reduced. The EL element lighting devices are, for example, utilized for lighting devices in portable electronic apparatus such as wrist watches and PHSs, lighting devices in other battery driven electronic apparatus as well as backlights in liquid crystal display devices such as for portable terminal devices and battery driven electronic apparatus.
(2) Background Art
Until now some products of the lighting devices for battery driven electronic devices such as wrist watches and portable electronic apparatus have used the EL elements. EL elements consume less electric power than ultra small sized incandescent lamps and semiconductor light emitting elements and are suitable for small sized and thinned devices, therefor the EL elements have been used for such products.
EL elements are ones which make use of the so called electro luminescence phenomenon, and among these EL elements a dispersed type EL panel in which a fluorescent substance used for EL is dispersed in a dielectric substance is structured into a capacitor form. In this form, a light emitting substance formed by dispersing a fluorescent material into a dielectric material is sandwiched between two pieces of electrodes. Such an EL panel is generally driven by a driving circuit which generates pseudo AC waves.
Namely, although these types of EL elements can not inherently be lighted by a DC signal, through repetition of driving by pulses and discharging electric charges charged for the EL element, an EL element in the form of a capacitor can be driven in a same manner as in an AC drive, which is indicated above as a pseudo AC drive.
FIG. 3 is an example of these types of EL element driving circuits.
Numeral 1 is a battery, numeral 2 is an EL element driving circuit and numeral 3 is an EL element (EL panel).
The EL element driving circuit 2 is constituted by a high voltage pulse generating circuit 21, a discharge circuit 22 and a control circuit 23. The high voltage pulse generating circuit 21 is a so called boosting circuit and is constituted by a transformer 24, a switching transistor 25, a diode 26 and an oscillator circuit 27 which receives signals from the secondary coil of the transformer 24 and performs a blocking oscillation.
The EL element driving circuit 2 receives an electric power supply from the battery 1 via the transformer 24, generates as an output of the transformer 24 high voltage pulses which are generated by switching (ON/OFF) an NPN type transistor (TR1) 25 inserted between the output side of the transformer 24 and the ground GND. And, driving circuit 2 charges and drives the EL panel 3 by applying the output to the EL panel 3 via a forward directed diode 26. Further, the EL panel 3, of which one electrode is connected to the side of the diode 26 and of which another electrode is connected to the ground GND, thus receives high voltage electric power. The discharge circuit 22 is constituted by an NPN type transistor (TR2) 28 connected between the EL panel 3 at the side where the diode 26 is connected and the ground GND and causes to discharge the electric charges in the EL panel 3 in a period other than the charging period.
The control circuit 23 drives the oscillation circuit 27 for a certain predetermined period and causes the transistor 25 by control signal A to perform a switching operation (ON/OFF) of a predetermined frequency, for example, several 10 kHz for a certain predetermined period to thereby generate an electric power of boosted high voltage. Thereafter, the control circuit 23 generate a control signal B and turns ON the transistor 28 to thereby discharge the electric charge in the EL panel 3. Namely, the control circuit 23 generates alternatively the control signal A and the control signal B at a certain cycle. Thereby, the timings of the charging period (a fly back pulse generation period) and the discharging period for the EL element are varied.
However, with such an EL element driving circuit, a variation in brightness of the EL elements is likely to occur product by product for the reason that the characteristic variation of respective elements directly affects onto the variation of their charging voltages. Therefore, it was determined to be necessary to insert a resistor circuit used for detecting the charging voltage at the output side as a load and to adjust the brightness as, for example, disclosed in JP-A-76-5952(1995). With such a measure the variation in brightness can be adjusted, however, with the provision of such a detection circuit, a power loss is correspondingly induced and the advantage of low power consumption for the lighting devices using EL elements is correspondingly reduced.